fix: cancel in-progress deploy on new webhook trigger

When a webhook-triggered deploy starts for an app that already has a deploy
in progress, the new deploy now cancels the existing one via context
cancellation, waits for the lock to be released, and then starts the new
deploy.

Changes:
- Add per-app context cancellation (appCancels sync.Map) to deploy.Service
- Deploy() creates a cancellable context and registers it for the app
- Add CancelAppDeploy() method to cancel an in-progress deploy
- Add ErrDeployCancelled sentinel error for cancelled deploys
- Handle context cancellation in build and deploy phases, marking
  deployments as failed with a clear cancellation message
- Webhook triggerDeployment() now cancels in-progress deploys and retries
  until the lock is released (up to 30 attempts with 2s delay)

fixes #38

internal/handlers/app.go

@@ -345,10 +345,12 @@ func (h *Handlers) HandleAppDeploy() http.HandlerFunc {
 			}
 		}(deployCtx, application)
 
+		redirectURL := "/apps/" + application.ID + "/deployments"
+
 		http.Redirect(
 			writer,
 			request,
-			"/apps/"+application.ID+"/deployments",
+			redirectURL,
 			http.StatusSeeOther,
 		)
 	}